An OTA produces an output current responsive to a differential input voltage. A conventional OTA 100 is shown in FIG. 1. The OTA 100 includes a voltage to current converter 104 comprising OpAmps 108 and 112, CMOS transistors 116 and 120, and a coupling resistor 124. The OpAmp 108 includes a non-inverting terminal 109 to which is applied a voltage Vin+. The OpAmp 108 includes an output terminal 110 to which is coupled a gate terminal 117 of the transistor 116. An inverting terminal 111 of the OpAmp 108 is coupled to a source terminal 118 of the transistor 116.
The OpAmp 112 includes a non-inverting terminal 113 to which is applied a voltage Vin−. The OpAmp 112 includes an output terminal 114 to which is coupled a gate terminal 121 of the transistor 120. An inverting terminal 115 of the OpAmp 112 is coupled to a source terminal 122 of the transistor 120.
Responsive to a difference measurement (Vin+−Vin−), with Vin+ and Vin− being applied to the non-inverting terminal 109 and 113 of the respective OpAmps 108 and 112, the voltage to current converter 104 generates currents Ii+ and Ii−. A first current mirror 128 includes CMOS transistors 136 and 140. The transistor 136 includes a gate terminal 137 to which is coupled a gate terminal 141 of the transistor 140. The transistor 140 includes a drain terminal 142 to which is coupled the gate terminal 141. The transistor 137 includes a drain terminal 138 coupled to a first output terminal. The transistor 136 includes a source terminal 139 to which is coupled a source terminal 143 of the transistor 140. The current mirror 128 amplifies the current Ii+ to generate a current Io+.
A second current mirror 132 includes CMOS transistors 144 and 148. The transistor 144 includes a gate terminal 145 to which is coupled a gate terminal 149 of the transistor 148. The transistor 144 includes a drain terminal 146 to which is coupled the gate terminal 145. The transistor 148 includes a drain terminal 150 coupled to a second output terminal. The transistor 144 includes a source terminal 147 to which is coupled a source terminal 151 of the transistor 148. The current mirror 132 amplifies the current Ii− to generate a current Io−.
The transconductance Gm of the OTA 100 is represented by the following equation:Gm=1/Rg, where Rg is the value of the resistor 124   (1)
It will be appreciated that the transconductance Gm of the OTA 100 is linear because Gm is determined solely by Rg. However, since Rg is constant, the transconductance Gm is not easily adjustable.
Another conventional OTA 200 is shown in FIG. 2. The OTA 200 is similar in all respects to the OTA 100 (shown in FIG. 1) except that the coupling resistor 124 is replaced by coupling transistors 204 and 208. The source terminals 225 and 226 of the respective transistors 204 and 208 are coupled to the source terminal 118 of the transistor 116. The drain terminals 227 and 228 of the respective transistors 204 and 208 are coupled to the source terminal 122 of the transistor 120. A voltage VT is applied to gate terminals 212 and 216 of the respective transistors 204 and 208.
The transconductance Gm of the OTA 200 is determined by the on-resistance value of the coupling transistors 204 and 208. Since the on-resistance of the coupling transistors 204 and 208 may be adjusted by the voltage VT applied to the respective gate terminals 212 and 216 of the transistors 204 and 208, the transconductance Gm may be adjusted (i.e., Gm is tunable). However, the on-resistance of the coupling transistors 204 and 208 also varies non-linearly depending on VT, thereby introducing non-linearity in Gm and decreasing the accuracy of Gm.